Endogenous opioids involved in opioidergic neurotransmission are peptides consisting of 5 or more amino acids. It is generally assumed that clearance of these peptides from the synapse and consequent termination of neurotransmission are mediated by peptidases. Recently, we have identified a novel sodium (Na)-and chlorine (Cl)-coupled transporter that is specific for opioid peptides. Such a transporter has not been described in the literature for this class of neurotransmitters. The expression of this transporter is demonstrable in the rat brain and in human neuronal cell lines. The characteristics of this transporter are similar to those of previously known other neurotransmitter transporters. We hypothesize that this transporter represents a new, hitherto unrecognized, component of opioidergic neurotransmission and hence a key determinant of opioid function and opiate addiction. Tat, the transactivator protein coded by the human immunodeficiency virus HIV-1, markedly up-regulates this transporter in cell lines. Since opioids exhibit immunomodulatory function and influence the pathogenesis and progression of AIDS, we predict that Tat-induced changes in the expression of the opioid peptide transporter may have clinical relevance to patients with HIV-1 infection. Even though the functional evidence for the existence of the transporter is unequivocal, nothing is known at present on the molecular nature of the protein responsible for the transport function. Here we propose to clone the transporter using functional expression strategy and establish the molecular identity of the transporter. We will then elucidate the functional features of the transporter using heterologous expression systems with mammalian cells and Xenopus oocytes. Successful cloning of the transporter will lead to the molecular identity of the gene and the protein responsible for the novel transport function. This will facilitate development of tools such as nucleotide probes and antibodies necessary to perform molecular studies on the expression and localization of the transporter in intact animals and in cultured cells. We will also compare the expression and activity of the transporter in control mice and in Tat-transgenic mice to assess the influence of HIV-1 Tat on opioid function in intact animals. These studies will lead to the discovery of a previously uncharacterized novel protein which might function as a key player not only in the area of opioid biology and opiate addiction but also in the progression of HIV-1 infection and AIDS in opiate addicts. [unreadable] [unreadable] [unreadable]